1. Field of the Invention
The present invention relates to an optical printer or scanner and, more specifically, to an electro-optical head assembly.
2. Discussion of Related Art
Electro-optical head assemblies are used in optical printers or optical scanners. In case of a printer, the electro-optical elements are usually formed by LED's for outputting the image information of a complete scanning line of an image to be printed, and the lens array comprises a plurality of red lenses or fiber-optical lenses (Selfoc lenses) arranged to focus the light emitted from the LED's onto a scanning line of a photo-sensitive member, such as a photoconductive drum. The number of lens fibers in usually smaller than the number of LED's, so that each fiber transmits the light of a plurality of LED's. Conversely, since the fiber-optical lenses are capable of producing an upright image, a plurality of neighboring lenses can contribute to the image of an individual LED. Accordingly, the positioning of the lenses relative to the LED's in a longitudinal direction of the array is not critical. However, the LED array and the lens array must be held strictly in parallel with each other and the distance between the two arrays must be maintained with high accuracy in order to obtain a good image quality.
Since the LED's dissipate a considerable amount of heat energy, the support member for the LED's is subject to thermal expansion. In view of this problem, U.S. Pat. No. 4,821,051 discloses a printhead of the type described above in which the support member has a heat sink structure and the LED's are mounted on a backing plate which is fixed to the heat sink in a manner to allow for differential thermal expansion of the heat sink and the backing plate. The LED array is composed of a plurality of tiles each carrying a large number of LED's, and the thermal expansion coefficient of the backing plate is matched to that of the tiles.
Support members for supporting the lens mount are provided on both longitudinal ends of the heat sink, and the lens mount, the support members and the heat sink are fixedly held together by screws. In addition, two elongate plate members are disposed between the heat sink and the frame on both sides of the LED array. The lower edge portions of these plate members are fixed on the heat sink with screws, and a gasket type foam material is interposed between the upper edges of the plate members and the lens mount, so that the LED's are contained in a dust-protected enclosure.
If an optical head assembly of this type is to be used for printing or scanning large-size documents, for example with the AO format, the overall length of the head assembly must be made so large that differences in thermal expansion between the lens mount and the support member for the electro-optical elements become significant and may lead to undesired distortions of the assembly. Further, with increasing length of the assembly, it becomes more difficult to provide a sufficient strength of the lens mount to prevent it from bending under its own weight or external forces.